Jack's Blowhole is actually a cave which runs from the Catlins coast inland for about 200m. It is filled with sea water and can not be entered. At high tides and during storms the water rushes into the narrowing cave and is forced out at the other end through an collapse of the ceiling. However, unlike other blowholes there is no fountain of sea water springing up, but there are roaring sound effects from both the water and the air expelling from the tunnel.
Jack's Blowhole is named after the famous Ngai Tahu Maori chief Hone (Jack) Tuhawaiki. He lived in the south during the first part of the 19th century.
[Thanks to daywalk: It was formed by the caving-in of a section of roof from a sea-cave, and was called Opito (the navel) by Maori. Tuhawaiki (called Bloody Jack because of a certain swearword he picked up from the whalers) was paramount chief of Otago and Southland from 1835- 44. On the walk to the blowhole you pass Tuhawaiki (Jacks) Island which is a scenic reserve. According to Pope's South Island Travel Guide, one of Te Rauparaha's scouting parties came south and were ambushed at Cannibal Bay by Tuhawaiki. Te Rauparaha's men returned to attack Tuhawaiki's hastily-constructed fortifications and drove the defenders up onto False Islet (the headland betweeen Cannibal and Surat Bays). Many of Tuhawiki's men were killed, and Tuhawaiki himself is said to have survived only by diving into the surf and swimming 10 km to Tuhawaiki Island.
Another thing to admire is the replanting around the blowhole. When we first visited the blowhole (mumble-mumble years ago) there were bare paddocks around it. Then Forest and Bird planted flaxes for shelter, followed by trees. And just look at it now! ]
In addition there seems to be a lot of contradictory information on the blowhole, with this as another alternative :
[From daywalk again: The Catlins are all Jurassic siltstone and sandstone. It's called the Murihiku terrane. Look at the geological map of Murihiku on the GNS website:
http://www.gns.cri.nz/Home/Our-Science/Energy-Resources/Geological-Mapping/Geological-Maps/1-250-000-QMAP/Map-downloads
You'll see that Jack's Blowhole is marked 'MJc', which is 'Catlins Group - closely jointed, fossiliferous, massive to thin bedded sandstone and sandy mudstone with tuff and plant beds'.
The blowhole was formed when the roof of a seacave just caved in. ]
The Blowhole is classified as a Karst Cave which means :
The forming of karst caves is very complicated and it is still a topic of geological research. But some basic aspects are very simple: water containing carbon dioxide CO2 is able to dissolve limestone. Unlike with salt and gypsum, water is not able to dissolve limestone without a little help. And this helper is carbon dioxide CO2. Carbon dioxide is a very common gas, you know it from sparkling water, softdrinks, and beer. They all contain it, it makes them bubbling. Natural water also contains CO2, but much much less, as it is normally not bubbling. Just fill some water in a bottle and after some hours you will see small gas bubbles at the glas of the bottle.
Carbon dioxide is in the the air, about 0.03% of our atmosphere is this gas. Animals and humans breathe air, consume (burn) the oxygen and exhale carbon dioxide. Many carbon dioxide is produced by biologic processes in the soil. When the rain water falls, it first absorbs some carbon dioxide from the atmosphere, but then, oozing through the soil, it absorbs much more biogene CO2.
In the water, the CO2 breaks up in two pieces, thus producing a (very weak) acid:
CO2 + H20=H2CO3=H+ + HCO3-
When this water gets in contact with limestone, the acid solutes the limestone.
H+ + HCO3- + CaCO3=Ca++ + 2 HCO3
This happens on the rock surface, just at the border between rock and soil. But when the limestone, which is originally water proof, contains cracks produced by tectonic forces - which is very common - the water oozes into the rock and starts to widen the cracks and solute caves inside the layers. With the small amount of CO2, the amount of soluted limestone is very small and it takes several ten thousand years or more to form a cave.
The limestone in the spring water in karst areas is very common and well known to the people living in those areas. If you heat this water (e.g. for making coffee, in the washing machine,...) the CO2 escapes and the limestone gets solid again. This process is similar to the forming of many speleothems in caves, like drip stones.
In order to log this cache you need to do the following :
1) Take a photo of you and your GPS at the platform with the water in the background.
2) Walk around the blowhole and using your track log determine the width and breadth of the blowhole and compare to the above dimensions.
3) Enjoy yourself
PLEASE NOTE: The walk can be quite strenious in places and we recommend a reasonable level of fitness. Strollers are not recommended as there are styles and steps and some very steep gradient hills. Suggest you take along sunblock, camera and hat.
Further information can be found here
PLEASE NOTE: Jacks Blowhole is closed for lambing in September and October